1. Field of the Invention
This invention relates to a venetian blind, and more particularly to a venetian blind that includes a cord-winding device that imparts a spring force to balance the total weight of a bottom rail and a covering unit so as to maintain the bottom rail at any desired position.
2. Description of the Related Art
U.S. Pat. No. 5,482,100 discloses a cordless, balanced venetian blind or shade with a consistent variable force spring motor, which can maintain a bottom rail at any desired position by the balance between a spring force and the total weight of the bottom rail and a covering unit. The covering unit includes a plurality of parallel slats and two lifting cords. The venetian blind has the following disadvantages:
(1) A drum unit consisting of a storage drum and an output drum is provided for mounting of a spring. This increases the volume of the venetian blind.
(2) Two lifting cords are stored on a cord spool, and therefore do not move smoothly.
(3) When it is desired to increase the length of the covering unit, it is necessary to replace the spring with a new one possessing a higher coefficient of elasticity, or to mount one or more additional drum units on a back wall of a bracket, thereby resulting in difficulties in increasing the length of the covering unit.